The present invention generally relates to scanning systems capable of reading bar codes. More particularly, the present invention provides a safety feature for disabling a scanning illuminator when the movement of the scanning mirror is inadequate (too slow or absent).
Some scanning systems of the laser (light amplification by stimulated emission of radiation) type focus a laser beam upon a motor-driven, rotating or dithering scanning mirror such that the laser beam forms a scanning pattern across a target bar code. The scanning laser beam is back-reflected to a photodetector, which determines the intensity of the back-reflected laser beam and outputs a current in proportion thereto. Thus a varying signal is output by the photodetector as the laser beam sweeps across a pattern of light and dark "bars" in a bar code.
Control circuitry controls the cooperation and coordination of the components (including the timing) and converts the photodetector output signal into useful form. Scanning speed is chiefly controlled by the speed of the motor.
The preferred embodiment of the present invention has a three-phase, direct current (DC) motor of the brushless type. Rather than using a commutator (for appropriately reversing the magnetic fields to "pull" the armature around its axis), varying signals (commutation logic) are applied to the appropriate windings to cause the magnetic fields to constantly change. The commutation logic is microprocessor controlled.
A three-phase motor is used instead of a single-phase motor since it provides for constant instantaneous power, and therefore a constant speed--an important feature for scanning operations.
An important safety consideration is to be able to shut down the laser if the the motor and thus scanning mirror cease operating, or operate below a certain speed. The result of motor malfunctioning is unnecessarily prolonged laser beam projection on the scanning surface, upon pattern mirrors in the scanner, and out of the scanner into free space.